In a two-way radio, the frequency bands for receive and transmit operations may overlap for the radio. In this case, when the radio transmitter is operative, signals from the transmitter may be coupled to the radio receiver, causing interference. Although interference can happen when the transmitter and receiver are operating on overlapping carrier frequencies, interference can also occur on interfering intermediate frequencies or baseband frequencies. Direct conversion radios are particularly susceptible to the latter problem. Differences between operational modes may result from the receiver architecture (e.g., superheterodyne), system design (e.g., separate frequency blocks for transmit and receive signals) or transmitter architecture (e.g., direct conversion or frequency offset).
For example, a device can be transmitting on the uplink carrier while monitoring the downlink carrier using the receiver on a nearby frequency, such as is possible in a multimode communication system. In those cases where the downlink frequency is close to the uplink transmission frequency, the communication device can actually interfere with itself. In other words, the transmit power of the device is picked up by, and interferes with, the receiver of the device. In addition, radio self-interference can occur in a Global Positioning System (GPS) wherein the time to correlate a received GPS system signal is long. In this case, the prior art solution is to mute the device transmitter during receiver correlation, which is not acceptable as it limits the use of the device.
In a direct conversion radio, low intermediate frequency (LIF), or very low intermediate frequency (VLIF) radio, a first frequency conversion is performed wherein the carrier RF frequency is demodulated at or near the baseband. Very precise frequency planning is required to implement direct conversion in a radio. Otherwise, intermodulating frequency by-products appear in the form of unwanted spurious signals in an operating passband or receive band of the radio. In particular, where a direct conversion or VLIF receiver operates simultaneously with a co-located transmitter with non-constant envelope modulation, any transmitter signals coupling with the receiver mixer will produce second-order distortion products. The amount of distortion is proportional to the squared-amplitude envelope of the interfering signal, and occurs in the intermediate frequency (IF), which translates to the baseband in direct conversion or VLIF receivers. This can occur even in case where the receiver and transmitter are not functionally related.
In addition, if a received signal level exceeds the operating range of the baseband circuitry of the radio, the receiver performance degrades as a result of the decreasing signal to noise ratio and receiver selectivity. This may occur when interfering signals are very strong compared to the desired on-channel signal and the baseband circuitry becomes saturated as a result of the overload. This results in the desired on-channel signal becoming desensitized. Therefore, it is necessary to limit the interference prior to the baseband circuitry and maintain signal levels within the baseband circuit's operating range. Filter portions of the baseband circuitry can reduce adjacent interference by allowing only the desired on-channel frequency to pass through. However, in direct conversion or VLIF receivers, filtering is of little use as the incoming signal prior to the baseband circuitry comprises the desired monitored signal as well as the interfering signal.
Prior art means currently employed to directly suppress the second order interference involve increasing the isolation between the transmitter and receiver, such as through separate antennas, careful radio architecture layout, increasing selectivity of the receiver, or increasing second-order performance of the mixer. However, the latter may entail reduced gain prior to the mixer, which tends to decrease sensitivity. These prior art means tend to increase cost, weight, size, or decrease performance of the receiver. Another prior art solution seeks to provide simple periodic calibration using DC levels in the IF. However, periodic calibration is still not suitable for the dynamic changes in interference that occurs when an antenna interacts with its surroundings in a mobile environment. In addition, significant DC accuracy is required and DC coupling is required. This is not compatible with direct conversion receivers which strip away the DC component. Still other prior art reject second order interference from a continuous envelope signal with low noise. This is not compatible with the new third generation carrier signals that are amplitude modulated, or those continuous envelope systems with high noise in the receiver IF.
As a result, there is a need for an improved method and apparatus to reduce second order transmit signal interference in a direct conversion, LIF or VLIF receiver. It would also be advantageous to provide this improvement without concern for the radio architecture circuitry layout or component improvements.